U.S. patent number 6,251,282 [Application Number 09/456,795] was granted by the patent office on 2001-06-26 for plasma filter with helical magnetic field.
This patent grant is currently assigned to Archimedes Technology Group, Inc.. Invention is credited to Richard L. Freeman, Tihiro Ohkawa, Sergei Putvinski.
United States Patent |
6,251,282 |
Putvinski , et al. |
June 26, 2001 |
**Please see images for:
( Certificate of Correction ) ** |
Plasma filter with helical magnetic field
Abstract
A plasma mass filter using a helical magnetic field for
separating low-mass particles from high-mass particles in a
multi-species plasma includes a cylindrical outer wall located at a
distance "a" from a longitudinal axis. Also included is a coaxial
cylindrical inner wall positioned to establish a plasma chamber
between the inner and outer walls. The magnetic field is generated
in this chamber with an axial component (B.sub.z) and an azimuthal
component (B.sub..theta.), which interact together with an electric
field to create crossed magnetic and electric fields. The electric
field has a positive potential, V.sub.ctr, on the inner wall and a
zero potential on the outer wall. With these crossed magnetic and
electric fields, a multi-species plasma is moved through the
chamber with a velocity, v.sub.z, high-mass particles in the plasma
(M.sub.2) are ejected into the outer wall and low-mass particles
(M.sub.1) are confined in the chamber during transit of the chamber
to separate the low-mass particles from the high-mass particles,
where M.sub.1 <M.sub.c <M.sub.2, and where M.sub.c =(ea.sup.2
(B.sub.z.sup.2 +B.sub..theta..sup.2)/8v){f(B.sub..theta. /B)}.
Inventors: |
Putvinski; Sergei (La Jolla,
CA), Ohkawa; Tihiro (La Jolla, CA), Freeman; Richard
L. (Del Mar, CA) |
Assignee: |
Archimedes Technology Group,
Inc. (San Diego, CA)
|
Family
ID: |
23814187 |
Appl.
No.: |
09/456,795 |
Filed: |
December 8, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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192945 |
Nov 16, 1998 |
6096220 |
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Current U.S.
Class: |
210/695; 204/554;
204/660; 209/12.1; 209/227; 210/222; 210/243; 210/748.01; 95/28;
96/2; 96/3 |
Current CPC
Class: |
B03C
1/023 (20130101); B03C 1/288 (20130101); H01J
49/328 (20130101) |
Current International
Class: |
H01J
49/30 (20060101); H01J 49/26 (20060101); B03C
001/00 () |
Field of
Search: |
;95/28 ;96/1,2,3
;209/12.1,222,722 ;210/222,223,243,695,748 ;204/554,660 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
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Rotating Plasmas in a Vacuum-Arc Centrifuge; Plasma Physics and
Controlled Fusion, vol. 29, No. 5, pp. 601-620; Great Britain,
1987. .
Bonnevier, Bjorn; Experimental Evidence of Element and Isotope
Separation in a Rotating Plasma; Plasma Physics, vol. 13; pp.
763-774; Northern Ireland, 1971. .
Kim, C.; Jensen, R.V.; and Krishnan, M; Equilibria of a Rigidly
rotating, Fully Ionized Plasma Column; J. Appl. Phys., vol. 61, No.
9; pp. 4689-4690; May, 1987. .
Dallaqua, R.S.; Del Bosco, E.; da Silva, R.P.; and Simpson, S.W.;
Langmuir Probe Measurements in a Vacuum Arc Plasma Centrifuge; IEEE
Transactions on Plasma Science, vol. 26, No. 3, pp. 1044-1051; Jun.
1998. .
Dallaqua, Renato Sergio; Simpson, S.W. and Del Bosco, Edson;
Experiments with Background Gas in a Vacuum Arc Centrifuge; IEEE
Transactions on Plasma Science, vol. 24, No. 2; pp. 539-545; Apr.
1996. .
Dallaqua, R.S.; Simpson, S.W.; and Del Bosco, E; Radial Magnetic
Field in Vacuum Arc Centrifuges; J. Phys. D.Appl.Phys., 30; pp.
2585-2590; UK, 1997. .
Evans, P.J.; Paoloni, F. J.; Noorman, J. T. and Whichello, J. V.;
Measurements of Mass Separation in a Vacuum-Arc Centrifuge; J. Appl
phys. 6(1); pp. 115-118; Jul. 1, 1989. .
Geva, M.; Krishnan, M; and Hirshfield, J. L. ; Element and Isotope
Separation in a Vacuum-Arc Centrifuge; J. Appl. Phys 56(5); pp.
1398-1413; Sep. 1, 1984. .
Krishnan, M.; Centrifugal Isotope Separation in Zirconium Plasmas;
Phys. Fluids 26(9); pp. 2676-2682; Sep. 1983. .
Krishnan, Mahadevan; and Prasad, Rahul R.; Parametric Analysis of
Isotope Enrichment in a Vacuum-Arc Centrifuge; J. Appl. Phys.
57(11); pp. 4973-4980; Jun. 1, 1985. .
Prasad, Rahul R. and Krishnan, Mahadevan; Theoretical and
Experimental Study of Rotation in a Vacuum-Arc Centrifuge; J. Appl.
Phys., vol. 61, No. 1; pp. 113-119; Jan. 1, 1987. .
Prasad, Rahul R. and Mahadevan Krishnan; Article from J. Appl.
Phys. 61(9); American Institute of Physics; pp. 4464-4470; May,
1987. .
Qi, Niansheng and Krishnan, Mahadevan; Stable Isotope Production;
p. 531. .
Simpson, S.W.; Dallaqua, R.S.; and Del Bosco, E.; Acceleration
Mechanism in Vacuum Arc Centrifuges; J. Phys. D: Appl. Phys. 29;
pp. 1040-1046; UK, 1996. .
Slepian, Joseph; Failure of the Ionic Centrifuge Prior to the Ionic
Expander, p. 1283; Jun., 1955..
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Primary Examiner: Reifsnyder; David A.
Attorney, Agent or Firm: Nydegger & Associates
Parent Case Text
This application is a continuation-in-part of application Ser. No.
09/192,945 filed Nov. 16, 1998, now U.S. Pat. No. 6,096,220. The
contents of application Ser. No. 09/192,945 now U.S. Pat. No.
6,093,320 are incorporated herein by reference.
Claims
What is claimed is:
1. A plasma mass filter with helical magnetic field for separating
low-mass particles from high-mass particles in a rotating
multi-species plasma which comprises:
a substantially cylindrical shaped outer wall defining a
longitudinal axis;
a substantially cylindrical shaped inner wall positioned coaxially
with said outer wall to establish a plasma chamber
therebetween;
a first magnetic means for generating an axial component of a
magnetic field (B.sub.z);
a second magnetic means for generating an azimuthal component of
said magnetic field (B.sub..theta.), said axial component (B.sub.z)
and said azimuthal component (B.sub..theta.) interacting with each
other to create said helical magnetic field;
means for generating an electric field substantially perpendicular
to said helical magnetic field to create crossed magnetic and
electric fields in said plasma chamber, said electric field having
a positive potential on said inner wall and a substantially zero
potential on said outer wall; and
means for injecting said rotating multi-species plasma into said
plasma chamber to interact with said crossed magnetic and electric
field for ejecting said high-mass particles from said plasma
chamber into said outer wall and for confining said low-mass
particles in said plasma chamber during transit therethrough to
separate said low-mass particles from said high-mass particles.
2. A filter with helical magnetic field as recited in claim 1
wherein said outer wall is at a distance "a" from said longitudinal
axis, wherein said inner wall is at a distance "b" from said
longitudinal axis, wherein said magnetic field has a magnitude
"B.sub.z " in an axial direction along said longitudinal axis and a
magnitude B.sub..theta. in an azimuthal direction around said
longitudinal axis, wherein said positive potential on said inner
wall has a value "V.sub.ctr ", wherein said outer wall has a
substantially zero potential, further wherein b has a value between
zero and 1, (0<b<1), and wherein said low-mass particle has a
mass less than M.sub.c, where
3. A filter as recited in claim 2 further comprising means for
varying said magnitude of said axial component (B.sub.z) of said
magnetic field relative to said magnitude of said azimuthal
component (B.sub..theta.) of said magnetic field.
4. A filter as recited in claim 2 further comprising means for
varying said positive potential (V) of said electric field at said
inner wall.
5. A filter as recited in claim 1 wherein said means for generating
said axial component of said magnetic field is a magnetic coil
mounted on said outer wall.
6. A filter as recited in claim 1 wherein said means for generating
said azimuthal component of said magnetic filed is a straight
conductor aligned on said longitudinal axis.
7. A filter as recited in claim 1 wherein said means for generating
said azimuthal component of said magnetic field is a plurality of
coils with each said coil being coplanar with said longitudinal
axis with a portion and each said coil having a portion of said
coil aligned substantially along said longitudinal axis.
8. A plasma mass filter for separating low-mass particles from high
ss particles in a rotating multi-species plasma which
comprises:
a cylindrical shaped wall surrounding a chamber, said chamber
defining a longitudinal axis;
means for generating a helical magnetic field in said chamber, said
magnetic field having an axial component (B.sub.z) and an azimuthal
component (B.sub..theta.);
means for generating an electric field substantially perpendicular
to said magnetic field to create crossed magnetic and electric
fields, said electric field having a positive potential on said
longitudinal axis and a substantially zero potential on said wall;
and
means for injecting said multi-species plasma into said chamber to
interact with said crossed magnetic and electric fields for moving
said multi-species plasma through said chamber in an axial
direction with an axial velocity v.sub.z, for ejecting said
high-mass particles into said wall, and for confining said low-mass
particles in said chamber during transit therethrough to separate
said low-mass particles from said high-mass particles.
9. A filter as recited in claim 8 wherein said wall is at a
distance "a" from said longitudinal axis, wherein said positive
potential on said longitudinal axis has a value "V.sub.ctr ",
wherein said wall has a substantially zero potential, further
wherein b has a value between zero and 1, (0<b<1), and
wherein said low-mass particle has a mass less than M.sub.c,
where
10. A filter as recited in claim 9 further comprising means for
varying said magnitude (B.sub.z.sup.2 +B.sub..theta..sup.2) of said
magnetic field.
11. A filter as recited in claim 9 further comprising means for
varying a current, I, through said magnetic field generating means
to control said velocity, v.sub.z, in accordance with the
expression; v.sub.z =10.sup.-7 eI/2M.sub.c.
12. A filter as recited in claim 9 wherein said multi-species
plasma is injected into said chamber at a distance r from said
longitudinal axis with 0.6a<r<a, and wherein said azimuthal
component of said magnetic field at the outer wall, B.sub..theta.a,
is such that B.sub..theta.a /B.sub.z <0.5.
13. A filter as recited in claim 9 wherein said means for
generating said axial component of said magnetic field is a
magnetic coil mounted on said wall.
14. A filter as recited in claim 9 wherein said means for
generating said electric field is a series of conducting rings
mounted on said longitudinal axis at one end of said chamber.
15. A filter as recited in claim 9 wherein said means for
generating said electric field is a spiral electrode.
16. A method for separating low-mass particles from high-mass
particles in a multi-species plasma which comprises the steps
of:
surrounding a chamber with a cylindrical shaped wall, said chamber
defining a longitudinal axis;
generating a helical magnetic field in said chamber, said magnetic
field having an axial component (B.sub.z) and an azimuthal
component (B.sub..theta.), and generating an electric field
substantially perpendicular to said magnetic field to create
crossed magnetic and electric fields, said electric field having a
positive potential near said longitudinal axis and a substantially
zero potential on said wall; and
injecting said multi-species plasma into said chamber to interact
with said crossed magnetic and electric fields for moving said
multi-species plasma through said chamber in an axial direction
with an axial velocity v.sub.z, for ejecting said high-mass
particles into said wall and for confining said low-mass particles
in said chamber during transit therethrough to separate said
low-mass particles from said high-mass particles.
17. A method as recited in claim 16 wherein said wall is at a
distance "a" from said longitudinal axis, wherein said positive
potential on said longitudinal axis has a value "V.sub.ctr ",
wherein said wall has a substantially zero potential, further
wherein b has a value between zero and 1, (0<b<1), and
wherein said low-mass particle has a mass less than Mc, where
18. A method as recited in claim 16 further comprising the step of
varying said magnitude (B.sub.z.sup.2 +B.sub..theta..sup.2) of said
magnetic field to alter M.sub.c.
19. A method as recited in claim 16 further comprising the step of
varying said positive potential (V.sub.ctr) of said electric field
at said longitudinal axis to alter M.sub.c.
20. A method as recited in claim 16 further comprising the step of
varying a current, I, to generate magnetic field with control over
said velocity, v.sub.z, in accordance with the expression; v.sub.z
=10.sup.-7 eI/2M.sub.c.
Description
FIELD OF THE INVENTION
The present invention pertains generally to systems and apparatus
which are useful for separating charged particles in a
multi-species plasma according to their respective mass. More
particularly, the present invention pertains to plasma mass filters
which rely on specially configured crossed magnetic and electric
fields, and on low collisionality between charged particles, to
eject high-mass particles from a plasma chamber while confining
low-mass particles in the chamber as the plasma transits through
the chamber. The present invention is particularly, but not
exclusively, useful for moving a multi-species plasma through a
plasma mass filter by generating an axial velocity for the
plasma.
BACKGROUND OF THE INVENTION
The general principles of operation for a plasma centrifuge are
well known and well understood. In short, a plasma centrifuge
generates forces on charged particles which will cause the
particles to separate from each other according to their mass. More
specifically, a plasma centrifuge relies on the effect that crossed
electric and magnetic fields have on charged particles. As is
known, crossed electric and magnetic fields will cause charged
particles in a plasma to move through the centrifuge on respective
helical paths around a centrally oriented longitudinal axis. As the
charged particles transit the centrifuge under the influence of
these crossed electric and magnetic fields they are, of course,
subject to various forces. Specifically, in the radial direction,
i.e. a direction perpendicular to the axis of particle rotation in
the centrifuge, these forces are: 1) a centrifugal force, F.sub.c,
which is caused by the motion of the particle; 2) an electric
force, F.sub.E, which is exerted on the particle by the electric
field, E.sub.r ; and 3) a magnetic force, F.sub.B, which is exerted
on the particle by the magnetic field, B.sub.z. Mathematically,
each of these forces are respectively expressed as:
F.sub.c =Mr.omega..sup.2 ;
F.sub.E =eE.sub.r ; and
F.sub.B =er.omega.B.sub.z.
Where:
M is the mass of the particle;
r is the distance of the particle from its axis of rotation;
.omega. is the angular frequency of the particle;
e is the electric charge of the particle;
E is the electric field strength; and
B.sub.z is the magnetic flux density of the field.
In a plasma centrifuge, it is universally accepted that the
electric field will be directed radially inward. Stated
differently, there is an increase in positive voltage with
increased distance from the axis of rotation in the centrifuge.
Under these conditions, the electric force F.sub.E will oppose the
centrifugal force F.sub.C acting on the particle, and depending on
the direction of rotation, the magnetic force either opposes or
aids the outward centrifugal force. Accordingly, an equilibrium
condition in a radial direction of the centrifuge can be expressed
as:
.SIGMA.F.sub.r =0 (positive direction radially outward);
F.sub.c -F.sub.E -F.sub.B =0;
It is noted that Eq. 1 has two real solutions, one positive and one
native, namely: ##EQU1##
where .omega.=eB.sub.z /M.
For a plasma centrifuge, the intent is to seek an equilibrium to
create conditions in the centrifuge which allow the centrifugal
forces, F.sub.c, to separate the particles from each other
according to their mass. This happens because the centrifugal
forces differ from particle to particle, according to the mass (M)
of the particular particle. Thus, particles of heavier mass
experience greater F.sub.c and move more toward the outside edge of
the centrifuge than do the lighter mass particles which experience
smaller centrifugal forces. The result is a distribution of lighter
to heavier particles in a direction outward from the mutual axis of
rotation. As is well known, however, a plasma centrifuge will not
completely separate all of the particles in the aforementioned
manner.
As indicated above in connection with Eq. 1, a force balance can be
achieved for all conditions when the electric field E is chosen to
confine ions, and ions exhibit confined orbits. In the plasma
filter of the present invention, unlike a centrifuge, the electric
field is chosen with the opposite sign to extract ions. The result
is that ions of mass greater than a cut-off value, M.sub.c, are on
unconfined orbits. The cut-off mass, M.sub.c, can be selected by
adjusting the strength of the electric and magnetic fields. The
basic features of the plasma filter can be described using the
Hamiltonian formalism.
The total energy (potential plus kinetic) is a constant of the
motion and is expressed by the Hamiltonian operator:
where P.sub.r =Mv.sub.r, P.sub..theta. =Mrv.sub..theta. +e.PSI.,
and P.sub.z =Mv.sub.z are the respective components of the momentum
and e.PHI. is the potential energy. .PSI.=r.sup.2 B.sub.z /2 is
related to the magnetic flux function and .PSI.=V.sub.ctr
-.alpha..PSI. is the electric potential. E=-.gradient..PSI. is the
electric field which is chosen to be greater than zero for the
filter case of interest. We can rewrite the Hamiltonian:
We assume that the parameters are not changing along the z axis, so
both P.sub.z and P.sub..theta. are constants of the motion.
Expanding and regrouping to put all of the constant terms on the
left hand side gives:
where .OMEGA.=eB/M.
The last term is proportional to r.sup.2, so if
.OMEGA./4-.alpha.<0 then, since the second term decreases as
1/r.sup.2, P.sub.r.sup.2 must increase to keep the left-hand side
constant as the particle moves out in radius. This leads to
unconfined orbits for masses greater than the cut-off mass given
by:
M.sub.C =e(Ba).sup.2 /(8V.sub.ctr) where we used:
and where a is the radius of the chamber.
So, for example, normalizing to the proton mass, M.sub.p, we can
rewrite Eq. 2 to give the voltage required to put higher masses on
loss orbits:
Hence, a device radius of 1 m, a cutoff mass ratio of 100, and a
magnetic field of 200 gauss require a voltage of 48 volts.
The same result for the cut-off mass can be obtained by looking at
the simple force balance equation given by:
.SIGMA.F.sub.r =0 (positive direction radially outward)
F.sub.c +F.sub.E +F.sub.B =0
which differs from Eq. 1 only by the sign of the electric field and
has the solutions: ##EQU2##
so if 4E/rB.sub.z.OMEGA.>1 then .omega. has imaginary roots and
the force balance cannot be achieved. For a filter device with a
cylinder radius "a", a central voltage, V.sub.ctr, and zero voltage
on the wall, the same expression for the cut-off mass is found to
be:
Where B=B.sub.z in this case, and when the mass M of a charged
particle is greater than the threshold value (M>M.sub.c), the
particle will continue to move radially outwardly until it strikes
the wall, whereas the lighter mass particles will be contained and
can be collected at the exit of the device. The higher mass
particles can also be recovered from the walls using various
approaches.
It is important to note that for a given device the value for
M.sub.c in equation 3 is determined by the magnitude of the
magnetic field, B, and the voltage at the center of the chamber
(i.e. along the longitudinal axis), V.sub.ctr. These two variables
are design considerations and can be controlled.
The discussion above has been specifically directed to the case
where the magnetic field is oriented substantially parallel to the
central longitudinal axis, and has only an axial component B.sub.z.
For the case wherein the magnetic field has a helical configuration
and, thus, has both an axial component, B.sub.z, and an azimuthal
component, B.sub..theta., a similar analysis leads to slightly
different result. The same derivation logic, however, still
applies.
To evaluate the effect of the azimuthal component, B.sub..theta.,
of the magnetic field on the cut-off mass, M.sub.c, one can use the
Hamiltonian formalism:
where P.sub.r, P.sub.z, P.sub..theta., are respective components of
canonical momentum, .PSI.=r.sup.2 B.sub.z /2 and A.sub.z
=B.sub..theta. rln(r) are components of magnetic vector potential
and .PHI.=V.sub.ctr -.alpha..PSI. is the electric potential. Taking
into account that the azimuthal and axial components of momentum as
well as total particle energy, H, are conserved, one can express
the radial component of the momentum, P.sub.r, as a function of
r:
where x=r.sup.2 /r.sub.o.sup.2, r.sub.o is initial coordinate of
the particle, .OMEGA.=eB/m is ion cyclotron frequency,
b=B.sub..theta. (r.sub.o)/B; B.sup.2 =B.sub..theta..sup.2
+B.sub.z.sup.2. As in the case of the standard filter the ion
orbits can be unconfined (Pr monotonically increases with r) or
confined (P.sub.r =0 at r>r.sub.o) depending on the ratio
M/M.sub.c which is defined by a mass of the ions. The additional
term in the last equation somewhat increases the cut-off mass which
can be described by the following approximate formula:
which has accuracy better than 1% in the full range of b.sub.1
0<b<1. If the ratio B.sub..theta. /B<1 then the cut-off
mass is not very sensitive to the Bo and hence to the initial
radial position of the ion in the filter. For example, if the
source of the plasma is limited by the radii, 0.6a<r<a, and
B.sub..theta. (a)/B.sub.z =0.3, one can expect variation of the
cut-off mass of about 10% which is acceptable for separation of the
ions with mass ratio of about 2.
It is also important to note that the addition of the azimuthal
magnetic field component, B.sub..theta., creates a controllable
axial plasma flow which has an axial velocity, v.sub.z, that can be
expressed as:
At E.sub.r.about.r, the axial velocity has a flat radial profile.
Further, the magnitude of the axial velocity, v.sub.z, is
proportional to the axial current, I, that is flowing in the
conductor or coil which generates the azimuthal component of the
magnetic field, B.sub..theta.. It can be mathematically shown that
this relationship is:
Accordingly, the axial velocity, v.sub.z, of plasma flow through a
filter can be controlled by variation of the current, I, that
creates B.sub..theta..
In light of the above it is an object of the present invention to
provide a plasma mass filter with a helical magnetic field which
effectively separates low-mass charged particles from high-mass
charged particles. It is another object of the present invention to
provide a plasma mass filter with a helical magnetic field which
has variable design parameters that permit the operator to select a
demarcation between low-mass particles and high-mass particles.
Still another object of the present invention is to provide a
plasma mass filter with a helical magnetic field which allows the
operator to control the axial velocity of the plasma through the
filter. Yet another object of the present invention is to provide a
plasma mass filter with a helical magnetic field which is easy to
use, relatively simple to manufacture, and comparatively cost
effective.
SUMMARY OF THE INVENTION
A plasma mass filter in accordance with the present invention
requires the generation of a helical magnetic field which is
crossed with an electric field in a low collisionality environment
to separate low-mass particles from high-mass particles in a
rotating multi-species plasma. More specifically, the plasma mass
filter of the present invention includes a cylindrical shaped outer
wall which is distanced from and coaxially oriented with a
cylindrical shaped inner wall to establish a plasma chamber between
the two walls. For purposes of disclosure, the outer wall is
located at a distance "a" from the common longitudinal axis, and
the inner wall is located at a distance "b" from the axis.
The helical magnetic field that is generated inside the chamber of
the plasma filter includes both an axial component (B.sub.z) and an
azimuthal component (B.sub..theta.). More specifically, the axial
component (B.sub.z) is generated by a series of magnetic coils that
are mounted on the outer wall. At the same time, the azimuthal
component (B.sub..theta.) is generated either by a straight
conductor that is aligned along the longitudinal axis of the
chamber, or by a plurality of coils which are each coplanar with
the axis and which have a portion of the coil aligned on the axis.
For the present invention, the axial component (B.sub.z) and the
azimuthal component (B.sub..theta.) interact with each other to
create the helical magnetic field.
The electric field that is generated inside the chamber of the
plasma filter is oriented to be substantially perpendicular to the
helical magnetic field. Thus, crossed magnetic and electric fields
are established in the plasma chamber. Importantly, the electric
field has a positive potential, V.sub.ctr, on the inner wall near
the longitudinal axis, and it has a substantially zero potential on
the outer wall. In the operation of the plasma mass filter of the
present invention a multi-species plasma, which includes both
relatively low-mass to charge particles (M.sub.1) and relatively
high-mass to charge particles (M.sub.2), is injected into the
plasma chamber to interact with the crossed magnetic and electric
fields. When M.sub.1 <M.sub.c <M.sub.2 and where M.sub.c
=ea.sup.2 (B.sub.z.sup.2 +B.sub..theta..sup.2)/8V.sub.ctr it will
happen that as the multi-species plasma transits the chamber, the
high-mass particles (M.sub.2) will be ejected from the plasma
chamber and into the outer wall. On the other hand, the low-mass
particles (M.sub.1) will be confined inside the plasma chamber
during their transit through the chamber. Due to their respective
interactions with the crossed electric and magnetic fields, the
low-mass particles are separated from the high-mass particles by
the plasma mass filter.
As intended for the present invention, the helical magnetic field
functions to establish an axial velocity, v.sub.z, for the
multi-species plasma as it transits through the plasma chamber.
This function, of course, also provides a "lift-off" effect for
drawing the multi-species plasma into the chamber in the first
instance. Control over the axial velocity, v.sub.z, is obtained by
varying the current, I, that is passing through the conductor
(coils) which creates the azimuthal component, B.sub..theta., of
the magnetic field. Specifically, the axial velocity, v.sub.z, can
be controlled in accordance with the expression; v.sub.z =10.sup.7
eI/2M.sub.c. Preferably, the current, I, will be in the range of
about thirty to forty KAmps d.c. (3040 KAmps).
DESCRIPTION OF THE DRAWINGS
The novel features of this invention, as well as the invention
itself, both as to its structure and its operation, will be best
understood from the accompanying drawings, taken in conjunction
with the accompanying description, in which similar reference
characters refer to similar parts, and in which:
FIG. 1 is a perspective view of the plasma mass filter according to
the present invention with portions broken away for clarity;
and
FIG. 2 is a perspective view of an alternate embodiment of the
plasma mass filter with portions broken away for clarity.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring initially to FIG. 1, one of the preferred embodiments of
the plasma mass filter in accordance with the present invention is
shown and generally designated 10. Another preferred embodiment of
the plasma mass filter is shown in FIG. 2 and is generally
designated 10'. In all important respects, the filter 10 and the
filter 10' are essentially the same. Accordingly, the components of
filter 10 and filter 10' are interchangeable unless suggested
otherwise herein.
As shown in FIG. 1, the filter 10 includes an open ended cylinder
12 which establishes an outer wall 14 for the filter 10. Further,
the outer wall 14 is oriented on a central longitudinal axis 16,
and the outer wall 14 is positioned at a radial distance "a" from
the axis 16. Additionally, the filter 10 includes a cylinder 18
which is positioned inside the cylinder 12 and coaxially aligned
with the cylinder 12 along the longitudinal axis 16. As shown, the
cylinder 18 has an inner wall 20 which is located at a radial
distance "b" from the axis 16 to establish a plasma chamber 22 in
the region between the outer wall 14 and the inner wall 20.
FIG. 1 also shows that the filter 10 includes a plurality of
annular shaped, coaxial electrodes, of which the electrodes 24a and
24b are representative. For the purposes of the present invention,
the electrodes 24a and 24b are oriented on the longitudinal axis 16
to generate the electric field E.sub.r. While it is shown in FIG. 1
that the electrodes 24a and 24b are located at the entrance end 26
of the plasma chamber 22, it will be appreciated that the
electrodes 24a and 24b could just as easily be placed at the exit
end 28 of the plasma chamber 22. Alternatively, electrodes 24a and
24b can be positioned at both of the ends 26, 28. Further, it will
also be appreciated that the electrodes 24a and 24b can be replaced
by a spiral electrode 30 (see FIG. 2). Like the electrodes 24a and
24b, the spiral electrodes 30 can be positioned at either or both
of the ends 26, 28. In any event, the purpose of the electrodes 24a
and 24b (or spiral electrode 30) is to establish a radially
oriented electric field, E.sub.r, in the plasma chamber 22.
Importantly, this electric field, E.sub.r, is established with a
positive potential, V.sub.ctr, on the inner wall 20 (i.e. near the
longitudinal axis 16) and a substantially zero potential on the
outer wall 14. Thus, the orientation of E.sub.r is in a direction
that is substantially perpendicular to the longitudinal axis 16.
Also, the electric field, E.sub.r, is directed radially outward
away from the axis 16.
Both FIG. 1 and FIG. 2 show that a helical configured magnetic
field is established in the plasma chambers 22 of both the filter
10 and filter 10'. In FIG. 1 and FIG. 2, the spiral path 32 is
representative of these magnetic fields and is shown to have an
axial component, B.sub.z, and an azimuthal component,
B.sub..theta.. In accordance with the present invention, the axial
component, Bz, for both the filter 10 and filter 10' is generated
by a plurality of coils, of which the coils 34a-d are
representative. As shown, the coils 34a-d are mounted on the
outside of the cylinder 12 and around the axis 16 to generate an
axial component, B.sub.z, which is substantially parallel to the
axis 16.
The azimuthal component, B.sub..theta., of the magnetic field in
the plasma chamber 22 can be generated in several ways. One way to
generate the azimuthal component, B.sub..theta., is to employ a
conductor 36 which is aligned along the longitudinal axis 16 inside
the cylinder 18 (see FIG. 1). A power source (not shown) which is
connected with the conductor 36 is then activated to run a current,
I, through the conductor 36 and thereby generate the azimuthal
component, B.sub..theta.. Another way to generate the azimuthal
component, B.sub..theta., is to employ a plurality of coils, of
which the coils 38a-d are representative (see FIG. 2). For this
particular embodiment of the present invention, it is preferable
that each of the coils 38a-d extend at least partially along the
axis 16. As shown in FIG. 2, the plane of each of the coils 38a-d
is substantially perpendicular to the respective planes of each of
the coils 34a-d, and vice versa.
In the operation of the filter 10 or filter 10' of the present
invention, a positive voltage, V.sub.ctr, is established on the
inner wall 20 and is controlled by either the annular shaped
electrodes 24a and 24b or the spiral electrode 30, depending on
which type of electrodes are used. Also, the coils 34a-d are
activated to generate the axial component, B.sub.z, of the magnetic
field, and a current, I, is generated in the conductor 36 (filter
10) or in the coils 38a-d (filter 10') to generate the azimuthal
component, B.sub..theta., of the magnetic field. Importantly, these
variables are established to satisfy the Eq.4 as set forth
above.
In accordance with earlier disclosure, when the configuration of
filter 10 or filter 10' is established as set forth above, a
multi-species plasma 40 can be introduced into the plasma chamber
22 through the entrance end 26. At this point it is to be noted
that although both FIG. 1 and FIG. 2 show the entrance end 26 below
the exit end 28, the entrance and exit into the plasma chamber 22
can be easily reversed. Indeed, in some instances it may be
preferable to reverse the entrance end 26 with the exit end 28 in
order to benefit from gravitational effects in the chamber 22, or
inject in the center of the device and remove light particles from
each end.
For purposes of disclosure, the multi-species plasma 40 will
typically include ions (charged particles) of different mass which
can be generally categorized as either low-mass to charge particles
42 (M.sub.1) or high-mass to charge particles 44 (M.sub.2).
Although the separation depends on mass to charge state, we will
use the convention low mass and high mass with the understanding
that multiple charged ions will have lower effective mass. Using
these categories, a relationship can be established in the plasma
chamber 22 wherein M.sub.1 <M.sub.c <M.sub.2. Consequently,
as the multi-species plasma 40 transits the filter 10 or 10'
through the plasma chamber 22, the high-mass particles 44 (M.sub.2)
will be ejected from the chamber 22 and into the outer wall 14
before completely transiting the filter 10 from entrance end 26 to
exit end 28. On the other hand, the low-mass particles 42 (M.sub.1)
are confined inside the chamber 26 as they transit through the
filter 10 (10') and emerge from the exit end 28. Thus, the low-mass
particles 42 (M.sub.1) are effectively separated from the high-mass
particles 44 (M.sub.2).
An important aspect of the present invention is that, due to the
helical configuration of the magnetic field, charged particles in
the multi-species plasma 40 are subjected to forces which will
cause the plasma 40 to move through the plasma chamber 22 with an
axial velocity, v.sub.z. It happens that this axial velocity is
controllable and can be established in accordance with the
expression: v.sub.z =10.sup.7 eI/2M.sub.c. In this expression for
v.sub.z, I is the current in the conductor 36 (filter 10), or the
coils 38a-d (filter 10'), and M.sub.c is determined as set forth
above in Eq. 4. Preferably, I will be in a range of about 30-40
KAmps. The practical effect of this control is that v.sub.z can be
established in such a way that the multi-species plasma 40 is more
easily drawn into the plasma chamber 22 for further processing.
Also, as stated above, v.sub.z assists in moving the multi-species
plasma 40 and, specifically, v.sub.z assists in the transit of
low-mass particles 42 (M.sub.1) through the plasma chamber 22.
While the particular Plasma Mass Filter with Helical Magnetic Field
as herein shown and disclosed in detail is fully capable of
obtaining the objects and providing the advantages herein before
stated, it is to be understood that it is merely illustrative of
the presently preferred embodiments of the invention and that no
limitations are intended to the details of construction or design
herein shown other than as described in the appended claims.
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